Prior art burglar alarm systems in the general field typically include a continuous conductive strip of soft tin/lead alloy foil applied to a window or screen. This creates a closed electric circuit which becomes open if the strip tears as a result of someone breaking the window or cutting the screen. A remote sensor and alarm detects the open circuit and signals an intrusion. Such systems provide a basic level of protection but there are inherent drawbacks.
First of all, the foil strip is extremely unsightly when applied to the window or screen and also alerts potential intruders of a protective system.
Second, the foil strip often fails to adhere to the window or screen surface as a result of long-term exposure to the sun's rays and moisture condensation.
Third, sophisticated intruders developed ways of avoiding detection by shunting the foil strip. This by-passes the closed-loop and allows the intruder to break the foil strip without setting off the alarm.
Finally, the foil strip generally could not be bent and therefore had to be pre-cut to span the area of the window or frame. This greatly increased the cost of manufacture and application of the strip.
U.S. Pat. No. 3,909,331 issued to Cohen attempted to solve the first of the foregoing problems by using pre-cut ornamental foil strips. These strips formed ornamental designs on the windows and frames. Although this may have been an improvement of sorts, it came at the cost of significantly higher manufacturing expenses. Moreover, the more serious prior art problems still existed.
More sophisticated alarm screens were developed such as those presented in U.S. Pat. No. 4,293,778 issued to Williams, and U.S. Pat. No. 4,146,293 issued to Mutten et al. These inventions included a continuous conductive strand interwoven throughout the screen mesh of a window screen.
These inventions were undetectable by outward appearance because the interwoven wire was indistinguishable from the surrounding screen mesh. However, the wires are very fragile and have little practical application. A more substantial (and hence, visible) wire is needed. In addition, manufacturing costs are extremely high. Furthermore, these alarm systems must be installed during the original manufacture of the window screen. They cannot be retrofit on a preexisting screen nor applied to any non-conducting standard screen (bug mesh) as is possible with the present invention.
U.S. Pat. No. 3,609,739 issued to Walter suggests a method of applying an electrically conductive lacquer strip to a pane of glass. The strip dries into an opaque closed circuit alarm path which triggers a remote alarm when the glass and lacquer is broken. This improvement yields a virtually undetectable alarm system which can be retrofit to existing windows. In addition, the conductive strip is relatively inexpensive to apply. However, the invention is specifically designed for application to a glass surface.
It would be preferable if an equally effective system could be installed in a window screen rather than glass pane. Since the screen can remain in place with the window open or closed occupants can freely open windows for fresh air without sacrificing protection.
Unfortunately the Walter improvement does not lend itself well to window screens. It is designed to be effective only when an intruder breaks the glass or attempts to shunt the circuit. The device relies on the travelling effect of cracks in glass. It relies on a sensitive thin lacquer strip which severs when interrupted by a travelling crack. This principle is inapplicable to screens. For application to screens there is a clear need for a "lacquer strip" or flexible conductive path which is capable of triggering an alarm when the screen is flexed, jimmied, or otherwise manipulated, in addition to being cut.
In accordance with the broad teachings of the present invention, there is herein, illustrated and described.